The present invention generally relates to an illumination system comprising a radiation source and a green-emitting ceramic luminescence converter. The invention also relates to a green-emitting ceramic luminescence converter for use in such an illumination system.
More particularly, the invention relates to an illumination system and a green-emitting ceramic luminescence converter for the generation of specific, colored light, including white light, by luminescent down conversion and additive color mixing based on an ultraviolet or blue radiation-emitting radiation source. A light emitting diode as a radiation source is especially contemplated.
Today, light emitting illumination systems comprising visible colored light emitting diodes as radiation sources are used either single or in clusters for all kinds of applications where rugged, compact, lightweight, high-efficiency, long-life, low-voltage sources of white or coloured illumination are needed.
Such applications comprise, inter alia, illumination of small LCD displays in consumer products such as cellular phones, digital cameras and hand-held computers. Pertinent uses include also status indicators on such products as computer monitors, stereo receivers, CD players, VCRs, and the like. Such indicators are also found in systems such as instrument panels in aircraft, trains, ships, cars, etc.
Multi-color combinations of pluralities of visible, colored LEDs in addressable arrays containing hundreds or thousands of LED components are found in large-area displays, such as full-color video walls and also high-brightness large-area outdoor television screens. Green, amber and red-emitting LEDs are increasingly being used for traffic lights or in effect lighting of buildings.
Conventional visible, colored-light emitting diodes, however, are typically subject to a low light output and are considered difficult to manufacture with uniform emission characteristics from batch to batch. The LEDs can also exhibit large wavelength variations across the wafer within a single batch as well as strong wavelength and emission variations resulting from operating conditions such as drive current and temperature. This applies especially to green-emitting LEDs.
Therefore, when generating white light with an arrangement comprising visible, colored-light emitting diodes, there has been the problem that white light of the desired tone cannot be generated due to variations in tone, luminance and other factors of the visible, colored-light emitting diodes.
It is known that visible, white or colored light illumination can be provided by converting the color of light emitting diodes emitting in the UV to blue range of the electromagnetic spectrum by means of a luminescent material comprising a phosphor.
Such phosphor-enhanced “white” LED systems are based in particular on the dichromatic (BY) approach, i.e. mixing yellow and blue colors, in which case the yellow secondary component of the output light may be provided by a yellow phosphor and the blue component may be provided by a phosphor or by the primary emission of a blue LED.
Likewise, white illumination systems are based on the trichromatic (RGB) approach, i.e. on mixing three colors, namely red, green and blue, in which case the red and green component may be provided by a phosphor and the blue component by the primary emission of a blue-emitting LED.
As recent advances in light-emitting diode technology have brought very efficient light emitting diodes emitting in the near UV to blue range, today a variety of colored and white-light emitting phosphor-converted light emitting diodes are on the market, challenging traditional incandescent or fluorescent lighting.
WO 2004036962 A1 discloses a light emitting device comprising a light emitting structure capable of emitting primary light of a wavelength less than 480 nm and a luminescent screen comprising a green phosphor of the general formula (Sr1-a-bCabBacMgdZne)SixNyOz:Eua, wherein 0.002≦a≦0.2, 0.0≦b≦0.25, 0.0≦c≦0.25, 0.0≦d≦0.25, 0.0≦e≦0.25, 1.5≦x≦2.5, 1.5≦y≦2.5 and 1.5<z<2.5.
The prior art phosphor-enhanced light emitting device typically utilizes an arrangement in which a semiconductor chip having a blue-emitting LED thereon is covered by a layer of epoxy resin containing pigment particles of one or more conversion phosphors. These phosphor particles convert the blue light to white or colored light, as described above.
However, a problem in prior art illumination systems comprising microcrystalline phosphor powders is that they cannot be used for many applications because they have a number of drawbacks.
First, the deposition of a layer of uniform thickness is difficult. Since color uniformity requires a uniform thickness, color uniformity is difficult to guarantee. In areas where the phosphor is thicker, the light appears more greenish white, while in sections having a thinner phosphor layer the light appears bluish white.
Second, these conventional phosphor particles are polycrystalline. Polycrystalline phosphor tends to be opaque. As a result, the phosphor particles absorb light, which lowers the light output. In addition, particles scatter the blue light, leading to a lower light-extraction efficiency. Third, the particles tend to agglomerate, and hence, providing a uniform layer with particles of a known size is difficult.